In this article we summarise the primary control methods for 6 degree of freedom (DoF) subsea electric manipulators and discuss which input devices are commonly used. Read on to find out how you can customise your own devices to control Reach Robotics’ dexterous manipulators.
Human Machine Interfaces (HMIs) allow operators to interact with complex systems by translating input signals into outputs which can be understood by the system. Operator inputs can be captured through many types of hardware including buttons, switches and joysticks – commonly found in devices such as keyboards, mice and gamepads. A good HMI is intuitive and easy to use, therefore selecting the right input device is important to achieving a smooth operator experience.
Subsea electric manipulators with 6 DoF can perform complex tasks because their dexterity allows for full translation in the XYZ axes and rotation in roll, pitch and yaw. This provides dexterity similar to that of a human arm, enabling operators to perform complex, high-precision tasks like turning valves or tying ropes.
Control Methods for 6 DoF Subsea Electric Manipulators
As a robotic manipulator’s number of degrees of freedom increases, the complexity of tasks it can perform also increases. However, this comes with increased control complexity – the user must provide more inputs and be able to do so in an intuitive manner to direct the robotic arm correctly.
6 DoF subsea electric manipulators are most commonly controlled via two methods: direct joint control, and kinematic control.
Direct Joint Control for Robotic Arms
With direct joint control, each input is mapped to a specific joint on the manipulator. This is suitable for situations when the exact position of each joint needs to be specified, with a precise goal pose in mind for both the end effector and intermediate joints. For example, there may be an obstruction in the workspace that the robotic arm needs to reach around to pick up an object – having control over each joint allows for avoiding unwanted collisions with the obstruction.
Kinematic Control for Electric Manipulators
With kinematic control, the operator inputs the desired position and orientation of the end effector, and the electric manipulator moves to achieve that position/orientation. This is suitable in situations where the arms exact path is not a priority, as long as the end effector reaches the desired endpoint. For example, an operator may want to interact with and rotate a valve in a relatively unobstructed environment – they can quickly position the end effector to achieve this goal in kinematic mode, without worrying about how every joint is positioned.
There are many hardware options suitable for translating operator inputs into subsea electric manipulator actions. Some are more suitable for direct joint control, and others more suitable for kinematic control. Take a look at the following devices for an idea of what solutions are available.
Master Arm Controller
The Reach Robotics 7-function Master Arm Controller has been designed to imitate the joint structure of a 6 DoF electric manipulator. The joints are mapped one-to-one – as the operator moves one Master Arm joint, the corresponding manipulator joint moves. The extra function in the “7-function” designation refers to the additional motion of the end effector jaws.
This imitative design results in intuitive control as the operator can dictate the exact position of every joint by visually replicating the desired pose using the controller. The Master Arm can be used with a single hand, however, two hands may sometimes be required if specific positions are desired for intermediate joints along the manipulator.
In addition to performing joint control, the Master Arm has a handle which offers additional inputs, featuring three buttons and a 2-axis joystick. Pressing the buttons results in the manipulator moving to preset poses, useful for deploying the manipulator into the workspace, or retracting and stowing it in a vehicle. The 2-axis joystick is used to open/close the grippers, and can also be mapped to the rotation of the wrist joint.
Command Pods
The Reach Command Pods are a hybrid of several input devices, featuring a 6-axis SpaceMouse controller, a 2-axis joystick, and 3 pushbuttons. The SpaceMouse can be used for kinematic control of the end effector, the joystick rotates the wrist and opens/closes the jaws, and the buttons move the arm into a number of preset pose. The compactness of the Command Pods allow them to be attached to the shoulders of a tablet controller, such as those used for piloting some inspection class ROVs, making them easy to integrate and use alongside an existing vehicle HMI system.
SpaceMouse®
A SpaceMouse is a 6-axis joystick that an operator can use to provide translation inputs in the XYZ axes, and rotation inputs in roll, pitch and yaw. Originally designed for navigating a 3D viewport in CAD software, the ability to move in 3D space translates well to controlling a manipulator end effector. In kinematics mode, the operator controls the pose of the end effector directly, and doesn’t have to think about how each individual joint is posed.
The SpaceMouse can have a bigger learning curve than other control devices that operators may already be familiar with, but it has the advantage of more precise control. It can be more intuitive than other kinematic control options, with the 6 DoF joystick mapping directly between the operator movements and the 6 DoF of the manipulator end effector, e.g. rolling the SpaceMouse causes the end effector to roll.
Various models of SpaceMouse feature additional input buttons, which can be mapped to manipulator functionality such as jaw operation and pose presets. Additionally, the SpaceMouse requires only a single hand for use, with no benefit from dual-hand operation, and it is a very physically compact device for the number of axes it has.
Gamepad
A gamepad (e.g. an Xbox controller) is traditionally used as input for a gaming console, but has found its way into the robotics field as a common input device. Popular among operators in Defence and Service Provider industries for its familiarity, gamepads have reduced the learning curve when operating electric manipulators. The wide array of available inputs (buttons, joysticks, triggers) means it is highly configurable.
Unlike the Master Arm which is only capable of direct joint control, and the SpaceMouse, which only provides kinematic control, the gamepad can be mapped to serve either purpose. Individual inputs can be mapped to joints, or directly to end effector degrees of freedom. With the right configuration, multiple arms, and even vehicles, can be controlled all from the same device.
The main limitation of the gamepad is that there is no single intuitive mapping for the 6 degrees of freedom, and no physical relationship between each input and the corresponding output. This results in a longer learning curve than for other devices, and possible operator confusion, despite being a familiar device.
Customised HMI Devices For Electric Manipulators
Any input device can be configured to control manipulators – it depends only on how intuitively the inputs can be mapped, and the specific needs of your application. If you find yourself wanting to integrate a custom device, you can utilise the Reach Robotics SDK to interface with our manipulators.
Master Pilot
The Master Pilot uses a visual-inertial tracking camera to track the device’s motion in 3D space, then translates these movements into manipulator commands via the SDK. As a result, the end effector mimics the operator’s hand motion intuitively. Please note, the Master Controller is not a commercially available Reach Robotics product – our R&D engineers just enjoy experimenting in-house!
Hand Tracking
This hands-free control option allows the operator to control a manipulator with a single camera using their hand movements. Specific gestures can be used to provide additional inputs, such as opening/closing the jaws. Please note, the Hand Tracking System is not a commercially available Reach Robotics controller, but was a successful internship project conducted in-house.
Which Input Device is Right For You?
Usability
Is the input device intuitive and easy to operate, and does it suit your desired control method (i.e. direct joint control or kinematic control)? If more than one subsea electric manipulator needs to be controlled simultaneously, consider whether the device can be operated effectively with a single hand.
Customisability
Consider if your scenario requires changing the configuration of an input device. If you need to re-map inputs to new outputs during an operation – you may need an input device and/or HMI that is sufficiently flexible and configurable.
Compatibility
Consider which input devices your HMI accepts, how the inputs will be communicated to the manipulator, and whether device behaviour is consistent across operating systems. For example, Reach Control supports Master Arms, SpaceMouse, and Gamepads in addition to the on-screen joint controls.
Hardware
Consider if your hardware has sufficient fidelity for its application, e.g. is your chosen joystick sensitive enough to provide a precise range of motion for an end effector? The input device should be suitable for the environment and rugged enough for its intended use. Consider durability, weight, and ease-of-use.
